docs: fix text decoration for file path

Author: kenjis

contributing/documentation.rst

@@ -21,7 +21,7 @@ To see the rendered HTML, ePub, PDF, etc., you will need to install Sphinx
 along with the PHP domain extension for Sphinx. The underlying requirement
 is to have Python installed.
 
-You can read more about installing all tools in /user_guide_src/README.rst
+You can read more about installing all tools in **user_guide_src/README.rst**
 
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